As you know "GIAO" is default and the NMR=GIAO doesn't work with the OPT key in a same time. In principle,no other option is required to run calculations of NMR chemical shifts.

It means that for NMR calculation the geometry is much more important and for any reason if the geometry changes, the chemical shift will be change.NMR it is notable: chemical shift calculations can currently only be performed with Cartesian basis functions, i:e: SPHERICAL=ON may not be specified .

I suggest you for using NMR=CSGT .The GIAO method, which uses basis functions that have an explicit field dependence, and the CSGT method, (developed by Keith and Bader) achieves gauge invariant by performing a continuous set of gauge transformations, one for each point in real space, obtaining an accurate description of the current density from which the shielding tensors can be determined.

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Hallo, Tomasz!

  I propose to increase the accuracy of SCF calculations (SCF=Conver=10 and possibly

 Integral=UltraFineGrid). The NMR calculations may be sensitive not only to the molecular geometry (and its accuracy, i.e. number of valid digits) but also to the start for SCF calculations (especially in transition metal complexes).

  Best wishes

                 mb

Thank you!